Quantum magnetism and quantum phase transitions

Abstract:

We report comprehensive inelastic neutron scattering measurements of the magnetic excitations in the 2D spin-5/2 Heisenberg antiferromagnet Rb2MnF4 as a function of temperature from deep in the Neel ordered phase up to paramagnetic, 0.13 < kBT/4JS < 1.4. Well defined spin-waves are found for wave-vectors larger than the inverse correlation length $\eta^{-1}$ for temperatures up to near the Curie-Weiss temperature, $\Theta_{CW}$. For wave-vectors smaller than $\eta^{-1}$, relaxational dynamics occurs. The observed renormalization of spin-wave energies, and evolution of excitation line-shapes, with increasing temperature are quantitatively compared with finite-temperature spin-wave theory, and computer simulations for classical spins. Random phase approximation calculations provide a good description of the low-temperature renormalisation of spin-waves. In contrast, lifetime broadening calculated using the first Born approximation shows, at best, modest agreement around the zone boundary at low temperatures. Classical dynamics simulations using an appropriate quantum-classical correspondence were found to provide a good description of the intermediate- and high-temperature regimes over all wave-vector and energy scales, and the crossover from quantum to classical dynamics observed around $\Theta_{CW}/S$, where the spin S=5/2. A characterisation of the data over the whole wave-vector/energy/temperature parameter space is given. In this, $T^2$ behaviour is found to dominate the wave-vector and temperature dependence of the line widths over a large parameter range, and no evidence of hydrodynamic behaviour or dynamical scaling behaviour found within the accuracy of the data sets.

Abstract:

Zero-field muon-spin relaxation measurements of the frustrated triangular quantum magnet AgNiO2 are consistent with a model of charge disproportionation that has been advanced to explain the structural and magnetic properties of this compound. Below an ordering temperature of TN=19.9(2) K we observe six distinct muon precession frequencies, due to the magnetic order, which can be accounted for with a model describing the probable muon sites. The precession frequencies show an unusual temperature evolution which is suggestive of the separate evolution of two opposing magnetic sublattices.

Abstract:

We report a high-resolution neutron diffraction study of the crystal and magnetic structure of the orbitally-degenerate frustrated metallic magnet AgNiO2. At high temperatures the structure is hexagonal with a single crystallographic Ni site, low-spin Ni3+ with spin-1/2 and two-fold orbital degeneracy, arranged in an antiferromagnetic triangular lattice with frustrated spin and orbital order. A structural transition occurs upon cooling below 365 K to a tripled hexagonal unit cell containing three crystallographically-distinct Ni sites with expanded and contracted NiO6 octahedra, naturally explained by spontaneous charge order on the Ni triangular layers. No Jahn-Teller distortions occur, suggesting that charge order occurs in order to lift the orbital degeneracy. Symmetry analysis of the inferred Ni charge order pattern and the observed oxygen displacement pattern suggests that the transition could be mediated by charge fluctuations at the Ni sites coupled to a soft oxygen optical phonon breathing mode. At low temperatures the electron-rich Ni sublattice (assigned to a valence close to Ni2+ with S = 1) orders magnetically into a collinear stripe structure of ferromagnetic rows ordered antiferromagnetically in the triangular planes. We discuss the stability of this uncommon spin order pattern in the context of an easy-axis triangular antiferromagnet with additional weak second neighbor interactions and interlayer couplings.